The present invention relates to an optical recording medium of write once type and a method for setting power of a laser beam for writing information in the recording medium.
As a write once type optical recording medium, a CD-R (write once type CD) is known. In the CD-R, a laser beam is focused on a recording surface to convert luminous energy to thermal energy to change the physical nature of the recording surface. Thus, the information is recorded n the CD-R.
Although the recording mediums of the same type are made by the same material, the individual recording medium has not always the same characteristic. Accordingly, if a laser beam is set to a fixed power, information can not be recorded in an optimum condition. Therefore, in such a recording medium, before recording information, the laser power is adjusted by an optimum power control (OPC) to be set to an optimum value for a disc to be recorded.
A conventional OPC method employed in the CD-R will be describe with reference to FIG. 6. The CD-R has a power calibration area (PCA) as a test area, read-in area, information recording area, and read-out area. The PCA is provided on the inside of the read-in area. A test writing is performed in the test area before recording information so as to obtain an optimum laser power for writing information on the CD-R.
In the write once type CD such as the CD-R, an area in which information has been written once is not used for further writing information again. A recorder for writing information is provided for controlling laser power by the test writing on the PCA, and for recording information on the information recording area thereafter with the controlled laser power.
As shown in FIG. 6, the CPA is divided into a hundred partitions. Since one writing test uses one partition, it is possible to perform a hundred times of the OPC.
However, the test writing over a hundred times can not be done. Therefore, even if a writable area (vacant space) remains in the information recording area, information can not be recorded thereon.